1. Field of the Invention
The present invention relates to a mobile terminal apparatus for performing radio communication with a base station and, more particularly, to a mobile communication terminal apparatus using an adaptive array antenna.
2. Description of the Related Art
An adaptive array antenna is an antenna which weights outputs from a plurality of antenna elements comprising an array antenna and arrayed in a predetermined shape by multiplying the outputs by weighting factors and can adaptively change directivity by controlling the weighting factors. Recently, a radio communication system with mobile communication terminals equipped with such adaptive array antennas has been under research and development.
FIG. 16 shows a typical example of a radio communication system using such mobile communication terminals. A mobile terminal 1000 adaptively controls weighting factors for a mounted adaptive array antenna 1010 to synthesize a beam pattern 1011 whose directivity is set toward a base station 1001 for the other party, thereby communicating with the base station 1001. This makes it possible to suppress transmission power and reduce power consumption as compared with a case where each mobile terminal is equipped with an omnidirectional antenna. When, therefore, this mobile terminal uses a power supply with a limited capacity, e.g., a battery, it can perform communication for a longer period of time. In addition, since radiation of interference power in other directions can be suppressed, interference with base stations other than the base station 1001 in communication can be suppressed.
In such a radio communication system, handover is performed when reception electric field strength necessary for the continuation of communication cannot be obtained as the mobile terminal 1000 gradually moves away from the base station 1001 for the other party. At the time of handover, it is required to synthesize a new beam pattern 1012 whose directivity is set toward a base station 1002 to which the mobile terminal 1000 is located close upon movement. To newly synthesize a beam pattern for the adaptive array antenna, a signal processing time is required for adaptive control on weighting factors. As a consequence, communication is interrupted for the signal processing time required for adaptive control accompanying the change from the beam pattern 1011 to the beam pattern 1012.
As described above, in a mobile communication terminal equipped with a conventional adaptive array antenna whose directivity characteristics can be adaptively changed, a signal processing time is required to adaptively control weighting factors for directivity synthesis for the adaptive array antenna. For this reason, communication is interrupted during the signal processing time required for adaptive control accompanying a change in beam pattern at the time of handover.